The present invention relates to paper-manufacturing machines and methods.
In particular, the present invention relates to a paper-manufacturing machine and method utilized to subject to suction a web or fiber-suspension layer which is supported by a felt or wire or other equivalent fabric which is guided around part of a roll of the machine.
Suction rolls are commonly utilized at the wet end of paper machines, which is to say in connection with the wire section and press section, for example as a sheet-forming roll, a couch roll, a pick-up roll, a felt-conditioning roll, and a press roll.
Known suction rolls conventionally include a rotating perforated cylindrical shell within which there is a stationary suction box extending parallel to the axis of the rotary shell, this suction box communicating through suitable seals with the inner surface of the cylindrical shell. Such a stationary suction box may have a width on the order of, for example, 100-500 mm, while extending in length from one end of the shell to the other end thereof. Such suction boxes are connected to a suction system in such a way that, when holes which pierce through the shell of the suction roll are in communication with the suction box, air flows through the holes which pierce through the shell into the suction box to achieve a suction at the exterior of the shell at that portion thereof which is in register with the suction box at any particular instant while the shell rotates.
The operation of such a suction box is that a wet paper web which has been formed at the sheet-forming section of the paper machine is conducted over the suction zone of the suction roll, while being supported either by a wire or felt, so that any vacuum which prevails at this location promotes the escape of water from the web through the holes into the suction box. Water extracted in this way from the web may travel through the holes as a result of the effect of the suction in the suction box, or the water may remain in the holes of the shell of the suction roll. In the latter event, water will remain in the holes in the shell as long as the holes are subjected to the effect of suction while air flows through the holes. However, the water is flung out of the shell of the suction roll when the holes turn beyond the suction zone.
The shell of such a conventional suction roll has a thickness of 50-100 mm, depending upon the dimensioning of the entire roll. The roll diameter and shell thickness are selected in such a way that deflection of the roll during operation of the paper manufacturing machine remains within permissible limits.
A conventional wire suction roll will have between 10,000 and 12,000 holes per square meter, and the diameter of each such hole is on the order of 5-6 mm. Suction rolls which are utilized in the press section of a paper machine have a larger number of holes in the shells thereof, but these holes have a smaller diameter, on the order of 4-5 mm.
Suction rolls are expensive components of paper-manufacturing machines. The drilling of the shells of the suction holes is particularly difficult, thus contributing to the high cost thereof. The perforations formed in the shell of a suction roll detract from the strength of the shell, and it therefore becomes necessary to utilize special alloys as raw materials for the suction rolls, as well as a considerable shell thickness, thus creating high material costs.
The air which enters into the suction box of the suction roll and which must be handled by the suction pump which is connected to the suction rolls originates from three sources:
(1) the air coming through the web,
(2) the air entrained into the suction zone along with the holes, in the interior of the latter, during each revolution of the suction roll,
(3) rogue air which enters the suction box as a result of seal leakage. This latter air is as a rule exceedingly minor representing only a small quantity, as compared to the first two sources referred to above. Thus, the major quantity of air received in the suction box is derived from the above first two sources.
The table following below illustrates the proportions between the first two air sources. The particular figures given refer to a suction roll in a particular paper machine, this roll having a length of seven meters and a suction box the width of which is 110 mm. The vacuum utilized is 550 mm Hg.
______________________________________ Through the web Machine speed (m/min) Hole air (m.sup.3 /min) (m.sup.3 /min) ______________________________________ 700 180 105 1000 260 &lt;105 ______________________________________
It is thus apparent from the above table that the air which arrives into the suction zone by way of the holes in the shell of the suction roll and which is carried into the suction system from these holes is unexpectedly high in modern, fast-operating paper machines. The higher the speed of operation of the machine, the greater will be the proportion of "hole" air. In other words the shell of the suction roll is formed with holes which pierce through the shell and which have air situated therein when these holes are situated beyond the suction box, and it is this air which is situated in the holes which is carried to the suction zone and drawn into the suction box and which represents an unusually high proportion of the air which is drawn into the suction box. This proportion of "hole" air is even further increased by the fact that with increasing machine speeds the rolls must be made of even greater strengths, and this greater strength is most often brought about by increasing the thickness of the shell of the suction roll. As a result the length of the holes piercing through the shell become longer and additional "hole" air is conveyed into the suction zone. Thus, the "hole" air quantity is proportional to the thickness of the roll shell.
In a particular newsprint machine having the speed of 1,000 m/min and a trimmed breadth of 8.5 m, the suction pump capacity required for handling the "hole" air, when considering all of the suction rolls combined, totals 108,000 m.sup.3 /hr, and the corresponding motor power which is required to drive the suction pumps is 2100 kW. If it is possible to reduce the suction pump power by 1000 kW, then there will be a saving of more than 7,000,000 kWh per year.
A further drawback encountered in operating techniques associated with conventional suction rolls is that the suction rolls generate loud noises, to the extent that such noise imposes severe health risks on the workers. It is possible to describe the manner in which this noise is generated. Thus, the holes in the shell of the suction roll act as whistles. As those holes which are subjected to vacuum travel beyond the suction zone, they are abruptly filled with air, and it is precisely at this point where the air rushes into the holes that a loud whistling noise is created, this loud whistling having a fundamental frequency equivalent to the acoustical resonating frequency of the hole. The multitude of numerous holes which are present in the suction roll created noise which frequently exceeds the pain limit of the human ear. Attempts have already been made to attenuate this noise by various arrangements such as by employing a suitable drilling pattern for the holes, but in practice no substantial attenuation of this noise has been achieved.
In connection with suction rolls at the press section, it is often essential to provide for deflection compensation, but this has not been possible heretofore because the space within the roll shell is already occupied by the suction box. As a result it has been impossible to accommodate any deflection-compensating means, which in themselves are known, in the hollow interior of the shells of section rolls in the press section.
Moreover, when in the press section use is made of grooved rolls which have at the exterior surface of the shell relatively wide grooves, such wide roll grooves create undesirable markings on the web.